Tree harvesters are of many different types. In general they include a vehicle, a movable arm or boom extended from the vehicle and a chain saw mounted to the distal end of the boom. The chain saw is mechanically and remotely maneuvered into position against the base of the tree and with a forceful sweeping motion the bar and chain cut into and through the tree trunk.
Chain saws, whether manual or tree harvester chain saws, have similarities in the manner by which they are mounted to the power head of the saw. The bar is elongated and in combination with a drive sprocket mounted to the power head of the chain saw defines an oval guide path for the saw chain. The guide bar is mounted to the power head with the bar's tail end adjacent the drive sprocket. The bar is initially mounted to be slidable toward and away from the drive sprocket. The chain is mounted onto the sprocket and guide bar and then the guide bar is slidably moved away from the sprocket to tension the chain. When the desired tension is achieved, the guide bar is clamped to the power head and thus at the desired position relative to the sprocket.
To allow for such sliding movement, the tail of the bar has an elongated mounting slot. In the manual chain saw, a pair of spaced apart bolts fixed to the power head protrude through the slot to keep the bar properly aligned but allow axial sliding of the bar. When the bar is properly tensioned, nuts threadably mounted to the bolts clamp the bar in place.
In a tree harvester, the force required to tension the much larger chain is frequently provided by hydraulic tensioners. In such cases, rather than bolts, the bar is mounted on a mounting apparatus including a base and a sliding block having a boss that fits into the mounting slot. The base is fixedly mounted to the power head and the block is slidably mounted to the base and is slid forward relative to the base and then clamped in place by hydraulics.
A problem for the harvester bar is the bending moment that is applied to the bar as the bar is forced through a tree trunk. Pivoting of the bar relative to the boom is resisted by the boss of the block that is fitted to the mounting slot of the bar and the applied force can literally split the bar. To prevent such splitting, the bar is provided with bolt holes at each side of the slot and the bar is additionally bolted to the sliding block. This solves the problem of the bar splitting but creates a mounting problem.
In order to replace the bar, the bolts have to be removed before the bar can be slid free of the boss. In mounting a replacement bar, following sliding of the bar onto the boss, the bolts have to be reapplied through the bolt holes in the bar and into the block.
The problem encountered is in the time it takes to unbolt and rebolt the bar for each bar change and even of greater concern is the human factor of simply not applying the bolts, e.g., as a result of carelessness, or on occasion a bolt or bolts are lost during the changeover (dropped in mud or snow) or simply due to hardship, e.g., an operator making the change over in 20 degree below zero weather intentionally leaving the bolts off.